Freshwater reservoir drawdown areas are an important yet not well understood source of atmospheric CO2. Quantifying CO2 emissions from drawdown areas is challenged by considerable spatial and temporal variability. In this study we aimed to assess temporal variability to understand the drivers of drawdown area CO2 emissions and to improve reservoir CO2 emission budgets. We measured CO2 emissions from the drawdown area of a German reservoir for three months with hourly resolution using automatic flux chambers. To analyze drivers of CO2 emissions we monitored sediment temperature and moisture as well as meteorological variables. For comparison we continuously measured CO2 emissions from the water surface by Eddy Covariance. CO2 fluxes showed pronounced diurnal and seasonal variability driven by temperature. Sediment moisture had a negative effect on CO2 fluxes – low fluxes were observed close to the waterline and after rain events. Inhibition of CO2 transport by water blocking sediment pores was indicated by a hysteresis between sediment temperature and CO2 flux, which was amplified under moist conditions. We show that daily CO2 emissions are overestimated by more than 25% if fluxes are measured only during midday. A comparison with CO2 emissions from the water surface measured by eddy covariance revealed that drawdown area and water surface contributed equally to the reservoirs CO2 emissions, despite the much small surface area of the drawdown area. Our study clearly shows that CO2 fluxes between reservoir drawdown area and atmosphere have a large diurnal variability in the same range as seasonal variability and within system variability. High temporal resolution measurements of CO2 emissions from the drawdown area are essential for precise quantification of greenhouse gas emissions from reservoirs.
Koschorreck et al. (Sun,) studied this question.